Cobalt chromium sulfide magnetic compositions having a pyrite type structure and devices containing same

ABSTRACT

MAGNETIC PYRITE-TYPE PHASES HAVING THE FORMULA WHERE X=0.01 TO 0.4. THE COMPOSITIONS ARE USEFUL IN MAGNETIC SWITCHING DEVICES, TEMPERATURE SENSING DEVICES, AND THE LIKE.

United States Patent 3,600,313 COBALT CHROMIUM SULFIDE MAGNETIC COM- POSITIONS HAVING A PYRITE TYPE STRUC- TURE AND DEVICES CONTAINING SAME Paul Christopher Donohue, Wilmington, Del., assignor to g. du Pont de Nemours and Company, Wilmington,

e No Drawing. Filed Oct. 31, 1969, Ser. No. 873,064 Int. Cl. C04b 35/00; H01v 3/00; H01b 37/32 US. Cl. 252-62.51 6 Claims ABSTRACT OF THE DISCLOSURE Magnetic pyrite-type phases having the formula C0 Cr 'S where x==0.0l to 0.4. The compositions are useful in magnetic switching devices, temperature sensing devices, and the like.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to magnetic pyrite-type phases having the formula Co Cr S where x is 0.01 to 0.4.

Description of prior art Chromium forms a series of sulfides from CrS to Crs which have crystalline structures related to those known in the art as the nickel-arsenide type. Cr S is ferromagnetic with a Curie temperature (T.,) of 303 K. There is no compound of the formula CrS The only compound known in the Co/Cr/S system is CoCr- 'S It has spinel-type structure and is ferromagnetic with T of 240 C.

C08 has been reported to have the pyrite-type structure and to exhibit metallic-type electrical conduction. It is ferromagnetic with a T of 118 K. and saturation magnetization (a of about 41.5 electromagnetic units (emu.) per gram. No substitution of Cr in C08 has been reported. However, when other transition metal sulfides, specifically FeS and NiS have been combined at ambient pressure with C08 to form solid solutions, the Curie temperature and saturation magnetization generally decrease to zero as the amount of iron or nickel is increased. (Both T and a increase slightly with initial iron substitution into C08 before decreasing to zero with further additions of Fe. Maxima for these two parameters occur at T -146 K. for Co Fe 'S and 1 43 emu./ g. for o.ss o.os 2-) DESCRIPTION OF THE INVENTION The compositions of this invention have the formula Co 'Cr S where x is 0.01 to 0.4. They have the pyritetype structure and are magnetic with the magnitude of T increasing with the Cr content and the magnitude of a decreasing with it. For example, when x=0.05, T =177 K. and 0 :36.53 emu./ g. while when x=0.25, T =343 K. and 0 :16.32 emu/g. The cubic cell dimension, a increases with increasing Cr content.

The phases are prepared by reaction of the elements or sulfides of Cr and Co mixed to produce the desired composition of the phase. To enhance reaction and impede the formation of Cr S excess S is useful in the reaction mixture.

The reagents should be thoroughly mixed. Reactions are carried out at 650 C. to 1300 C. and 1 atmosphere to 65 kilobars (kb.). Preferred conditions are 1100 C.- 1200 C. and 30 to 65 kb.

For high pressures and temperatures, a tetrahedral anvil pressure device as described by E. C. Lloyd et al., J. of Research, National Bureau of Standards, 630, 59

3,600,313 Patented Aug. 17, 1971 ice Bismuth I II 25.37.:002 kb. Bismuth II III 26.96i0.18 kb. Thallium II- III 36.69:0.11 kb. Barium II- III 590:1.1 kb.

All compressions are made on the cold assembly and the charges then heated to the desired temperature, measured with an appropriate thermocouple. No pressure correction for thermocouple behavior has been introduced; standard electromagnetic force (EMF) tables for one atmosphere were used. The temperatures are measured at the center of the external surface of the boron nitride crucible. The temperature at the ends is approximately 30% cooler.

The reaction conditions employed generally involve a slow cool "to enhance crystal growth. The crystals generally grow at the ends of the pellet in the form of black rods. in such cases separation from excess S or excess Cr S was done mechanically. Samples were washed with carbon disulfide to remove further excess sulfur.

The products of all reactions were examined by Debye- Scherrer X-ray powder techniques. The cubic unit cell dimension of the pyrite-type phase was refined using a computerized least squares technique in which the Nelson- Riley function is one of the parameters. When single-phase material was separable from a mixture, the compositions were determined by atomic absorption techni ue in some cases and X-ray fluorescence technique in others. When single-phase products were directly obtained, the composition was assumed to be equal to the reactant composition. The unit cell data and the analytical data obtained by atomic absorption were employed using Vegards rule (L. Vegard, Z. Phys. 5, 17 (1921); Z. Krist. 67, 239 (1928) --see also Concise Chemical and Technical Dictionary, H. Bennett, Chemical Publishing Co., 1962, i.e., when two similar crystalline materials form a solid solution, the lattice constant of this solution divides the space between their respective lattice constants in ratio to their relative quantities) to obtain a unit cell dimension for hypothetical CrS where a=5.593i0.008 A. This value was used to obtain the approximate composition of other phases by using Vegards rule.

The Cr content is a function of the starting ratio of elements and the pressure. When 30 kb. was used, a maxi-' mum value of x-0.l was obtained; however, at 65 kb. a maximum value of x-0.4 was achieved. When atmospheric pressure was used, the value of x was about 0.01.

Magnetic measurements were made using a vibrating sample magnetometer. Measurements were made of magnetization, (1, vs. field, 'H, and vs. temperature, T K. The Curie temperature, T saturation magnetization 0' in emu./ g. were obtained from the data. As x was increased, the value of T increased while a decreased.

These phases exhibit utility in many of the usual applications of magnetic materials such as magnetic switching devices, information storage, transformer coils, etc. They are also useful in applications requiring a range of temperature sensitivity or magnetization since these materials can be made to have such a range when the Cr content is varied.

Electrical measurements were made by a four-probe technique on crystalline samples.

The materials are electrical conductors with resistivities, p, in the range 10- to 10- ohm-cm. at room temperature. The variation of resistivity with temperature is very small. One sample was measured from 4.2 K. to 600 K.

The product contained black regions at the ends of the pellet and plastic sulfur in the center. The black region consisted of large and small crystalline regions. The larger crystals were a pyrite-type phase where a=5.540 A. The

and the resistivity varied from 4X 10' ohm-cm. to 5 smaller crystals were Cr S The composition of the pyrite- 5 l0 ohm-cm. This feature of small change in retype phase derived by Vegards rule is Co Cr 8 resistivity wlth a large change in temperature is desired and useful in some resistor applications. EXAMPLE 4 SPECIFIC EMBODIMENTS OF THE INVENTION om ms z The following are non-limitative examples of the in- 10 A mlxture of Part Part C0 and two Parts vention. Unless otherwise stated, all parts and ratios are Sulfur pelleted Pressured to 65 and reacted atomic weight ratios for cobalt metal and elemental sulat 1100 for W hours and quenched The product was fur and molecular weight ratios for chromium coma.homo.geneous Smgle Phase matenal' The X'ray Powder pounds diffraction pattern showed only a pyrite-type phase of EXAMPLE 1 5 cell dimenslon in which a=5.5422 A. Analysis of the Cr and Co content was done -by atomic absorption and o.'15 '0.25 z lndlcates the formula to be Co Cr S By Vegards A pellet weighing 0.3872 g. was pressed from a mixthe fermula 15 0.87 0.13 2- ture of Co metal powder, Cr S and elemental sulfur Maghetle measurements were made: "5 gmixed in the ratio 221:6. It was pressured to 65 kb. and and c= ;eacted at 1100 C. for one hour, cooled over a four EXAMPLE 5 our period to 900 and quenched. The product was composed of black, shiny material part of which was Co0'908Cr0'92 S2 magnetm The magnetic part was Separated by passing r A pellet was pressed from a mixture of 0.35 part CrS, the powdered sample through a magnet. X-ray diffraction part i Parts Pressured to 65 and of this sample showed a pattern similar to that of pyritereacted at 13.00 for two hours and quenched The type 0052, but the lines were Shifted to lower angles product consisted of end and center regions. The end indicating a larger cell. Diffraction lines of excess Cr S reglons were. composed of black crystals part of which were also in the pattern. The cell was measured and a magnetic at room temperature and Part were was found to equal 5.5495 A. This value is larger than h below h temperature" The former I" Showed the cell dimension of C08 in which 11:5 .5346 A. humanly a pyntetype Powder Pattern but some a s Using Veganrs rule, the composition of the pyrite type was also present. The latter part showed a pure pyritephase was calculamd to be cooflscrmssw type pattern where a=5 .5393 A. The composition calculated by Vegards rule is Co Cr S EXAMPLE 2 The center region of the pellet contained pyrite-type COOGCYMSZ phase and Cr S A pellet weighing 0.3782 g. was pressed from a mixture EXAMPLE 6 of 2 parts Co metal powder, one part Gr s and six parts crmficro 0552 sulfur. It was pressured to 65 kb. and reacted at 1300 C. for one hour and cooled over a period of two hours A Pellet Weighing 0-431 gmade from a miXtulB f to 1200, and quenched. Four regions of black crystalline 36 g- CO, 0063 g. Cr, and 0.862 g. S was pressured material were seen in the pellet. At the very end of the to 65 and heated to 13009 C: held for One 110111 at pellet, material which was magnetic at room temperature 00 C., ooled at a ra e of 50 per hour for four hours was present. The X-ray powder patterns showed the presto 1100 C. and quenched. The bulk of the product conence of Cr S and a pyrite-type phase of ll dime ion sisted of microcrystalline material. At a region close to :5,544 A, N xt to rd th enter, crystals of Cr S the ends of the pellet, larger crystals were separated mewere present. In the center of the pellet another magnetic chanically. The powder diffraction pattern of these indiregion was present. The powder pattern showed primarily cated a single phase material of pyrite-type structure with a pyrite-type ha e =5,558 A.) and a trace of Cr S 41:5.5376 A. According to Vegards rule, the composition The composition of this phase determined using Vegards is co cr s Magnetic measurements made on these rule is Co Cr S Magnetic measurements showed crystals show T =177 K. and 0 :36.53 emu./ g. T =275 K. and o =18.6 emu./g.

Around the center of the pellet another region was EXAMPLES Presenh Whleh was composed of @253 and a Poorly A series of reactions was run starting with various ratios erystalhhe Pynte'tyPe Phase' of CrS, Co and S. These were pressed into pellets, pres- EXAMPL 3 sured to 65 kb., and reacted at 1200 C. for two hours and quenched. The products of all reactions consisted of coe-elcro-oese black, rod-like crystals of the pyrite-type phase at the A mixture of 1 part Co metal powder, one part Cr ends of the pellet. In the center, the excess S or Cr S metal powder, and five parts sulfur, was pelleted, preswere present. The crystals were easily separable for sured to 30 kb. and reacted at 1200 C. for one hour, chemical analysis, magnetic and electrical measurements. cooled over a period of three hours to 900, and quenched. The data are shown in Table I.

TABLE I Magnetic data Resistivity Cell Methodof dimension determia. Starting ratio in Composition nation emuJg. To, K. pm x. pm x.

. O.5CrS/0.5CO/2S 5.5502 (2) CooJaCrumSz (a) 16.2 342 3.0 10- 3.1x10- 8 0.4ors/0.5oo/2S 5.5452 (2) COojzttCfoJ'mSg (b) 20.75 302 9 o.35ors/0.65o0/2s 5.5494 (1) oomormsr (a) 5.5456 (1) COo.s1CIo.raS2 (a) 10 0.25CrS/0.75CO/2S 5.5477 (1) co om sg (a) 5.5503 (1) CouJa om i: 11 0.20CrS/0.80CO/2S 5.5450 (1) C00 a35Cl'0,1u5Sg (c) 12 0.150rS/0.85GO/2S 5.5451 (1 co cr st (c) 1 (a) =Vegards rule; (b) =X-ray fluorescence; (c) =Atomic asborption. I Numbers in parenthese are standard deviations.

EXAMPLE 13 \A mixture of 0.05 Cr (0.0468 g.) and 0.95 Co (1.0078 g.) was dissolved dilute H 80 (1.5 ml. H SO4+20 ml. H 0) and nitrogen bubbled through the solution which was heated to speed reaction. The solution was poured into acetone to precipitate the metal sulfates in finely divided and intimately mixed form. The latter mixture was heated to 100 C. in vacuum to remove water of crystallization, then heated at 350 C. in a flowing HgS/Nz mixture to produce an intimate mixture of fine particle sulfides. This mixture was sealed in vacuo in a silica tube with a large excess of sulfur and heated at 650 C. for three days and quenched in cold water. Spectroscope analysis of the product showed 210% Cr to be present.

'X-ray powder dilfraction showed a good pyrite pattern with trace amounts ot Cr S The cell dimension is 5.5353 A. which corresponds approximately to the composition Co Cr S as derived using Vegards rule. Magnetic measurements showed T =140 K.

EXAMPLE 14 A mixture of 0.33 part Cr and 0.667 part Co was prepared in the manner described in Example 13. Excess sulfur was added to produce stoichiometric Co Cr S The mixture was sealed in a gold tube and heated for six hours at 3000 atm. pressure and 700 C. and then cooled. The product was extracted with carbon disulfide to remove unreacted sulfur and gave a black powder. The product was magnetic when cooled to liquid nitrogen temperature. The lDebye-Scherrer pattern showed the presence of a pyrite phase (11:5.5350 A.), Cr S and unidentified material. The cell dimensions of the phase indicate some Cr substitution, and had an approximate composition using Vegards rule of Co cr S 6 EXAMPLE A The product of Example 5 which was magnetic below room temperature was used in a thermomagnetic switching device. The end region of the pellet which showed a pure pyrite-type pattern of cell constant a=5.5393 A. which corresponds to composition C Cr S was used. It was [mounted using silver paste to a brass spring positioned opposite to a conducting contact and the assembly placed between the poles of a horseshoe magnet. An ohm meter was placed in series with the spring and the conducting contact.

When the device was cooled below room temperature the product became magnetic and the force of the magnetic material overcame the force of the spring, thus causing contact. Current flowed through the circuit as indicated by the ohm meter. The compounds of the invention are therefore useful as temperature sensing agents and may be used to construct switching devices for various purposes, i.e. thermostats, fire alarms, and the like.

What is claimed is:

1. A magnetic composition of the formula Co Cr S where x=0.0l to 0.4 having the pyrite-type structure.

2. The composition of claim 1 wherein x=0.25.

3. The composition of claim .1 wherein x=0.4.

4. The composition of claim 1 wherein x=about 0.09. '5. The composition of claim 1 wherein x=.1.3.

6. An electrical switching device containing a composition of claim 1 as a temperature sensing agent.

References Cited UNITED STATES PATENTS 3,372,997 3/1968 Bither 23315 TOBIAS E. LEVOW, Primary Examiner J. COOPER, Assistant Examiner US. Cl. X.R. 23-134; 335-146 

